This invention relates to an apparatus for heat treating a particulate commodity which operates on the fluidised bed principle, and more particularly but not exclusively relates to apparatus for heat-treating particulate commodities such as food products. The invention also relates to a method of heat treating a particulate commodity.
Various types of apparatus are available for roasting food products such as nuts and coffee beans. The most common is a perforate, or sometimes a non-perforate, rotating drum which contains the product and to which heat is applied by means of open-flame burners. Internal paddles are fitted to the drum to stir the product during roasting. Such an apparatus is unsatisfactory for treating large quantities of product. First, it operates on a batch basis only; second, substantial degradation of the product occurs due to abrasion as the drum rotates; and third it depends highly on the experience of the operator to develop the right flavour and odour components.
Another known arrangement comprises a perforate, slowly rotating cylinder. The cylinder has a helical vane on its inner wall surface, the vane extending inwardly by about half the radius of the cylinder and its function being to move the product along the cylinder. Hot air is blown from above to pass through the cylinder and hence through the product bed, which becomes heaped up to cover one of the lower quadrants of the cylinder wall, because of the rotational motion. Such an apparatus does allow continuous, as opposed to batch processing, but tends to produce uneven roasting throughout the product which is heaped up against the cylinder wall into a bed varying in thickness. This bed presents less resistance to air flow in its thinner regions, and hence the product in such regions is roasted more highly. Other disadvantages include the need for careful positioning of sealing means adjacent the outer surface of the cylinder wall at the circumferential limits of the bed to ensure all the hot air introduced into the cylinder passes through the product bed without escaping through either side. Furthermore, in such an apparatus, degradation of the product occurs due to abrasion, leading to spoiling of the product and clogging of the perforations in the cylinder wall. The apparatus is also complicated because of the need for extraction means, arranged below the cylinder, to remove husks and broken parts of the particulate product.
In order to avoid such degradation, perforate continuous belt arrangements have been used in which hot air is blown through a perforated belt on which the product is supported. Although this apparatus avoids the problem of degradation and additionally operates on a continuous basis, it is still very difficult to achieve uniform roasting of the product particularly across the width of the belt. In addition, in order to prevent the perforated mesh belt from becoming clogged with oil released from the product, flames are played on the mesh belt during its return run. This this introduces a substantial fire risk because of loose particles such as nut husks which are blown through the recycled air.
In another apparatus using hot air to heat the product, the air is blown through the mesh sidewall of a vertical tower and the product is loaded into the tower at the top and passes downwardly through a system of baffles under gravity to be extracted from the base. In this arrangement, the air is blown through a filtering system attached to the mesh sidewalls. This results in the necessity for frequent cleaning of the filters to avoid impairing the performance of the oven. Furthermore, due to the heat gradient across the tower, the product tends to cook faster on the side adjacent the heat source and attempts to reduce the thickness of the tower to overcome this problem reduces the throughput capability of the apparatus.
A more satisfactory apparatus is one which, for part of its length, combines the principle of the fluidised bed with a rotary drum. In this equipment, heated air is injected into the interior of a rotary drum which is provided with a helical inner surface and a longitudinal channel through which the product is moved from one end to the other. The distribution of the surfaces within the chamber draws the product through the drum and relatively uniform roasting is achieved by the hot air which is injected into the product at the first, dehydration, stage and then into the space above the product at two subsequent roasting stages.
Despite the fluidised bed principle of operation of this apparatus, some degradation of the product does occur by abrasion, and the dust which is produced during rotation can clog the apparatus and oxidise to contaminate the final product. Furthermore, because the product is not fully fluidised, a relatively high temperature of about 240.degree. C. is needed, with consequential high energy requirements. Conventional fluidised beds have been used for dehydrating food products, and they have a number of advantages compared with some other systems. With the particulate material in a fluidised condition, each particle is completely and continuously surrounded by a stream of drying or cooling relatively high velocity fluid, giving intimate contact between the solid particles and the fluid to reduce film resistance to evaporation and to ensure good heat transfer. In an ideally uniform gas-fluidised system, because of the high degree of solids mixing, there is a reduction of temperature gradients throughout the bulk of the bed to negligible values, i.e. the bed has a high effective internal thermal conductivity, and consequently all the particles theoretically receive the same heat treatment. If the particles have a range of sizes, the larger particles will tend to settle to the bottom of the bed, helped by a vibratory action of the deck along which the bed of particles is moved. Thus, the larger particles will be in the region of the higher velocity fluid, such as air, and heat transfer will be increased for these particles. Compared with other apparatus, due to the air stream, the particles suffer very little abrasion.
Nevertheless, a conventional fluidised bed does suffer, in practice, from the disadvantages of inconsistent results, due to difficulties in controlling conditions within the bed, such as the exact degree of heat applied to the bed along its length, and in controlling the dwell time of the product in a fluidised bed adapted for continuous processing of the product.
Although attempts to improve uniformity throughout the fluidised bed have been made by moving the fluidised particles along the bed using a conveyor belt to try and control the dwell time and/or stirring the particles to reduce segregation into stagnant areas and bubbles, the results have not been fully satisfactory. The conveyor belt has either hampered fluid flow to the bed and/or caused undue abrasion of the fluidised particles.
As a result of these difficulties, generally speaking, fluidised beds have been successfully used only for the dehydration stage of food products; they have not been successful for completing the entire roasting operation where precisely-controlled and consistent results are required. Yet a fluidised bed oven offers the advantage of cooking a product quickly, with a minimum loss of volatiles.